Recently, in the semiconductor industry, advanced processing techniques, such as CVD, dry etching, or gas cluster ion beam processing in a vacuum environment, are commonly used in the semiconductor manufacturing process. It is extremely challenging, if not nearly impossible, to use a vacuum chuck to hold a semiconductor wafer or other substrate in a vacuum chamber. Some contemporary solutions to this problem generally include mechanical holding systems that hold substrates made from semiconducting materials, typically silicon or silicon-containing materials, at their periphery. However, silicon wafers, for example, are extremely brittle and there is always some risk of tiny pieces chipping off of the semiconductor, or other substrate when using a mechanical holding system. Generating such small dust particles from the chipping may result in serious problems for the quality of production and potentially affect yield rate.
Semiconductor processing equipment, therefore, has increasingly relied upon the use of electrostatic clamping methods for holding substrates in place while processing, rather than mechanical clamping methods. The advantages of using electrostatic clamping methods generally include fewer particles being generated and, in some cases, simplified clamping hardware. In the effort to reduce particles in the vacuum, it is also desirable to have as few in-vacuum connections and components.
Contemporary electrostatic chuck designs are of either DC or AC configurations, and generally comprise one, two, or more poles. The chucks typically comprise a dielectric ceramic layer, or similar dielectric material, with the poles comprising a conductive material just below the clamping surface. High voltages are applied to a single pole, or pole-to-pole, relying on field changes in the dielectric layer effecting opposite field changes in the substrate, resulting in electrostatic forces to hold the substrate to the chuck.
The clamping force is directly proportional to the dielectric constant and the net pole voltage difference, and inversely proportional to the dielectric thickness. Therefore, the thinner the dielectric, the higher the clamping force. As higher through-put demands require higher-speed scanning, higher inertial forces are generated requiring higher clamping forces. Thus, it is desirable to have a chuck with the thinnest dielectric possible. However, one tradeoff to a thin dielectric is the voltage breakdown through it.
With AC chucks, a sinusoidal voltage waveform will have to have a peak voltage of about 1.4 times the desired clamping voltage in order to attain the same average force if DC was used instead. The peak-to-peak voltages required for AC chucks can become problematic for chucks with thin dielectrics as the peak voltages necessary for required clamping forces approach the breakdown voltages of the dielectric. DC chucks also often require phase reversal and decaying AC fields in order to discharge the net field, otherwise the substrate would never de-chuck.
With DC chucks, contemporary power supplies require an external floating signal super-imposed onto the high-voltage chuck signals, using frequency-to-voltage techniques to create a substrate present signal in order to test for presence of a substrate on the chuck. This technique requires extra components and sensors in the vacuum, which are undesirable as they are a potential source of particles in the vacuum. Also, this technique may not work with an AC output.
What is needed therefore is a power supply for an electrostatic chuck that can provide AC power to an AC chuck without the concern of the peak-to-peak voltage and be able to detect the presence of a substrate on either an AC or a DC chuck.